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U-II was initially isolated from the neurosecretory system of the Goby fish (Gillichthys mirabilis).[13] For many years it was thought that U-II does not exhibit significant effects in mammalian systems; a view quickly overturned when it was demonstrated that Goby U-II produces slow relaxation of mouse anococcygeus muscle, in addition to contraction of rat artery segments. In 1998, the genes for Pre-pro U-II were found in mammals proving that the peptide U-II did exist in mammals.[14]

The U-II gene is located on chromosome 1p36.[15] U-II peptide length varies between species due to the specific cleaving sites located at different spots depending on the species. In humans U-II length is 11 amino acids. The peptide sequence that is needed for biological function for both U-II and urotensin II-Related Peptide (URP) is known as the core. It is hexapeptide (-CYS-TYR-LYS-TRP-PHE-CYS-), and is connected at the two ends by a disulfide bond. Also just like URP the amino terminus can be modified without any loss in pharmacological activity suggesting that it is not needed for activation of the receptor. Unlike URP, U-II has an acidic amino acid (Glutamic or Aspartic) that precedes the core sequence. While the amino acid isn’t necessary for the activation of urotensin II receptor the fact that it is conserved in different species suggests that it has a biological function that has not been discovered.[14]

U-II when injected icv in rats also leads to cardiovascular responses including raising mean arterial pressure (MAP) and causing tachycardia. When the arcuate nucleus, and the paraventricular nucleus, two different areas of the brain which are known to control blood pressure were injected with U-II simultaneously they caused an increase in blood pressure. When the two areas were injected separately it was discovered that U-II affected the excitatory neurons in the paraventricular nucleus and the inhibitory neurons of the arcuate nucleus.[18]

U-II when injected icv in both rats and mice also stimulates locomotion in familiar environments. This experiment was also tested in rainbow trout (Oncorhynchus mykiss) where a stimulatory effect was also observed..

Depression-like behavior was also observed when U-II was injected in the brain by using the forced swim test and the tail suspension test which are used to compare molecules that are thought to cause anti-depressive-like effects.

Orexigenic behavior which is increased appetite and thirst were also observed after icv injection of U-II in rats.[18]